It's been one of my major pet peeves on both Android and iOS: the total and utter lack of consistency. Applications - whether first party or third party - all seem to live on islands, doing their own thing, making their own design choices regarding basic UI interactions, developing their own non-standard buttons and controls. Consistency died five years ago, and nobody seems to care but me.

I totally agree. In fact I often prefer to miss some functionality to installing another application that would look, feel and behave alien. The unholy mess of custom UI elements several times even forced me to uninstall the app right after first run.

In fact I was even considered building a custom GNOME-based firmwares for my phone and tablet, though I'm quite confident that I don't have enough time and knowledge to make it usable.